CA1125082A - Electrostatographic imaging with polyvinyl butyral coated carrier particle - Google Patents

Abstract

ABSTRACT

Electrostatographic developer mixtures comprising finely-divided toner particles electrostatically clinging to the surface of carrier particles comprising a core having an outer coating thereon comprising a polyvinyl acetal. The coated carrier particles have negative triboelectric charging properties and are partricularly useful in development systems, employing negatively charged photoconductive surfaces. Imaging processes are also disclosed.

Description

S~32 ELECTXOSTATOGKAPHIC IMAGIN~ WITH
.
POLYYINYL ACETAL COATED CARXIER PA~TICLE

This invention is generally concerned with electrostatographic 5 imaging systems and, more specifically, to improved imaging materials, their manufacture, and use in the development of electrostatic latent images.
It is well known to form and develop images on the surface of photoconductive materials by electrostatie methods such as described, for example, in U.S. Patents 2,297,691; 2,277,013; 2,551,5~2; 3,220,324; and lû 3,220,833. In summary, these processes as described in the aforementioned patents involve the formation of a charged electrostatic latent image on an insulating electrophotographic element and rendering the latent image visible by a development step whereby the charged surface of the photoconductive element is brought into contact with a developer mixture. As described in U.S.
15 Patent 2,297,691, for example, the resulting electrostatic latent image is developed by depositing thereon a finely-divided electroscopic material referred to in the art as toner, the toner being generally attracted to the areas of the layer which retain a charge thus forming a toner image corresponding to the electrostatic latent image. Subsequently, the toner image ean be trans 20 ferred to a support surface such as paper and this transferred image can be permanently affixed to the support surface using a variety of techniques including pressure fixing, heat fixing, solvent fixing, and the like.
Many methods are known for applying the electroscopic particles to the latent image including cascade development, touchdown and magnetic 25 brush development as illustrated in U.S. Patents 2,618,552; 2,895,847 and 3,245,823. One of the most widely used methods is cascade development wherein the developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the elec-30 trostatic latent image-bearing surface. ~agnetic brush development is also known and involves the use of a developer material comprising toner and magnetic carrier particles which are carried by a magnet so that the magnetic field produced by the magnet causes alignment of the magnetic carriers in a brush-like configuration. Subsequently, this brush is brought into contact with 35 the electrostatic latent image-bearing surface causing the toner particles tobe attracted from the brush to the electrostatic latent image by electrostatic attraction, as more specifically disclosed in U.S. Patent 2,8~4,063.
Carrier materials used in the development of electrostatic latent images are described in many patents including, for example, U.S. Patent 3,590,D00. The type of carrier material to be used depends on rs~any factors 5 such as the type of development used, the quality of the development desired, the type of photoconductive material employed and the like. Generally, however, the materials used as carrier surfaces or carrier particles or the coating thereon should have a triboelectric charging response commensurate with the triboelectric eharging value of the toner in order to generate 10 electrostatic adhesion of the toner to the carrier. Further, carrier materials should be selected that are not brittle so as to cause flaking of the surface orparticle brealc-up under the forces exerted on the carrier during recycle as such causes undesirable effects and could, for example, be transferred to the copy surface thereby reducing the quality of the final image.
There have been recent efforts to develop carriers and particularly coatings for carrier particles in order to obtain better development quality andalso to obtain a material that can be recycled and does not cause any adverse effects to the photoconductor. Some of the coatings commercially utilized deteriorate rapidly especially when employed in a continuous process whereby 20 the entire coating may separate from the carrier core in the form of chips orflakes as a result of poorly adhering coated material and fail upon impact and abrasive contact with machine parts and other carrier particles. Such carrier particles generally cannot be reclaimed and reused and usually provide poor print quality results. ~urther, the triboelectric values of some carrier coatings 25 have been found to fluctuate when changes in relative humidity oceur and thusthese carriers are not desirable for use in electrostatographic systems as they can adversely affes~t the quality of the developed image.
In addition, in particular electrostatographic reproduction systems in order to develop a latent image comprised of negative electrostatic charges, 30 an electrostatic carrier and toner powder combination must be selected in which the toner is triboelectrically charged positively relative to the granularcarrier. Likewise, in order to develop a latent image comprised of positive electrostatic charges such as where a selenium photoreceptor is employed, an electroscopic toner powder and carrier mixture must be selected in which the 35 toner is triboelectrically charged negatively relative to the carrier. Thus, where the latent image is formed of negative electrostatic charges such as , . :, : ~' ' ~, ~
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~5~2 when employing an organic electrophotosensitive material as the photore-ceptor, it is highly desirable to develop the latent irrage with a positively charged electroscopic powder and a negatively charged carrier material.
Further, for a given toner-carrier pair, the magnitude of the tribo-5 electric charge is important in that, if the charge is too low, the developedcopy will be characterized by high print density but resolution will be poor and bac~ground areas will be overdeveloped. By the same token, if the tribo-electric charge is too high, background areas will not contain unwanted de-posits and resolution will be good, but the developed image print density will 10 be too low. Therefore, for a toner-carrier pair to be satisfactory, the carrier material must be one wherein its coating has a high resistance to abrasion and good adhesion properties, it must be capable of triboelectrically charging the toner to the desired polarity, and charge the tosler to Q level within a range of triboelectric values whereby developed copies of high quality are obtained.
PRIOR ART
The development of electrostatographic coated carrier materials has been to a main extent on a trial and error basis. Since certain desirable functional characteristics of carrier materials having coatings are known, it has been somewhat possible to select coating materials for the purpose of determining their potential usefulness as carrier coatings. However, it has not been possible to reliably predict whether any particular coating material is viable or possesses any advantage over another coating material for use as an electrostatographic carrier coating. The only effective means of determining whether any coating material will satisfy the functional characteristics of a

2 5 carrier material is to actually prepare carrier particles with the coating material and evaluate the product. Thus, sven if a coating material is a member of a broad class OI generally desirable materials, it may be that the particular coating material is difficult or impossible to coat by normal means or requires a complicated or expensive preparation process.
One such class of generally desirable carrier coating materials is the class of fluoropolymers as disclosed in U.S. Patent 3,798,167 to Kukla et al.
In said patent, carrier particle cores are coated with a substantially insolublefluoropolymer or a mixture of a fluoropolymer and a modifying resin in which the fluoropolymer is essentially insoluble. As the fluoropolymer therein, that is, polytetrafluoroethylene or a copolymer thereof is substantially insoluble, it is applied to the carrier cores by first preparing a suspension or dispersion of the coating material. After coating the carrier cores, the coating material is heated to a temperature sufficiently high, that is, about 400C., so that the coating will ~e cured and fused as to adhere to the cores. ~s will be appreciated, such a coating procedure is multi-step7 time-consuming and more expensive than standard coating processes such as dipping or spray-drying a coating solution. Further, after treatment at the temperature required to fuse the fluoropolymer coating to the carrier cores, it is found that the carrier particles possess more negative triboelectric charging characteristics than the untreated coated particles. Also, inasmuch as it would be desirable to provide a carrier particle coated with a negatively charging coating, but provide such a coated carrier particle by a single-step coating operation, such a need exists.
- An aspect of the invention is as follows:
An electrostatographic imaging process comprising providing an electrostatographic imaging member having a recording surface, followed by forming a negative-ly charged electrostatic latent image on said surface,contacting the image with a developer mixture consisting essentiall~ of finely-divided toner particles electro-statically clinging to the surface of carrier particles, the toner particles consisting of a resin and colorant, and the carrier particles consisting of a core selected from the group consisting of iron, steel, ferrite, magnetite, nickel, and mixtures thereof, and having an average diameter of from between about 30 microns and about l,000 microns, said core containing thereon an outer coating of polyvinylbutyral in a thickness of from about 0.2 microns to about 0.7 microns, thereby resulting in a negative triboelectric charge on the carrier particles, and a positive triboelectric charge on the toner particles, causing at least a portion of the finely-divided positively charged toner particles to be attracted to and deposited on the recording surface in conformance with ~he electro-static latent image to provide a developed image, trans-ferring the developed image to a suitable substrate, and permanently affixing the developed image to said substr~ate.

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The polyvinyl butyral used in the practice of this invention is commercially available from Monsanto Plastics and Resins, St. Louis, Missouri under the trade mark FO~MVARo It has been found that the carrier coating materials hereinbefore described provide electrostatographic coated carrier materials which possess desirable negative triboelectric charging properties, excellent copy print quality, and life performance superior to known negatively charging coated carrier particles such as carrier particles coated with halogenated polymers.
When employing the polyvinyl acetal coating compo sitions on electrostatographic carrier core particles, it has been found that the thus coated carrier particles possess va~tly improved life performance characteristics over known halogenated polymer coating materials. Although not wishing to be bound by an explanation therefor, it is believed that the improved life performance characteristics of the carrier compositions of this invention are due to the outstanding adhesion and film forming properties of the coating materials. Such improved life per-formance characteristics of the carrier materials is especiallynotable when these polyvinyl acetals are applied to metallic carrier cores, since typically, halogenated resins applied to metallic carrier cores are unstable as evidenced by short carrier life. In addition, the coatiny compositions that may be used in the practice of this invention have been found to provide an especially desirable and useful range of tribo-electric charging properties to the carriex materials when employed in developer mixtures for the development of electro-static latent images formed of negative charges. Further, the negative triboelectric charging values of these poly~inyl ace-tal coated carrier particles is completely unexpected when employed with finely-divided toner particle compositions con-taining triboelectric charge control additives and result in improved performance in the development of negatively charged electrostatic latent images.
The polyvinyl acetal carrier particle coating com-positions are formed by the well-known reaction between alde-hydes and alcohols. Typically, the addition of one molecule ~25~
-5a-of an alcohol to one molecule of an aldehyde produces a hemi-acetal which is inherently unstable. However, hemiaceta~ are further reacted with another molecule of alcohol to form a stable acetal. In like fashion, polyvinyl acetals are pre-pared from aldehydes and polyvinyl alcohols. Polyvinylalcohols are usually classified as partially hydrolyzed, that ; is, containing 15 to 30% of polyvinyl acetate groups, and : .
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completely hydrolyzed, or containing 0 to 5% of polyvinyl acetate groups.
Both types, in various molecular weights, may be employed in producing com-mercial polyvinyl acetals.
In synthesis, the conditions of the acetal reaction and the con-5 centration of the particular aldehyde and polyvinyl alcohol used are closelycontrolled to form polymers containing predetermined properties of hydroxyl groups, acetate groups, and acetal groups. The product obtained may be represented by the following generic structural formula wherein the propol~
tions of A, Bg and C are controlled and are randomly distributed along the 10 molecule.

~/cH2\~ ~r 1 ~CH2--C
~ OH ~ ~ C--o A ~ C
Polyvinyl Acetal Polyvinyl Alcohol Polyvinyl Acetate As earlier indicated, these ma terials are commercially available 1~a cler~ k s 20 from M[onsanto Plastics and Resins, Sto Louis, Missouri under variousl~e-~4S such as Butvar and Formvar. Number designations have been given for these commercial compositions and provide a summary indication of the molecular nature of the polymer. For exarmple, the first digits of the Formvar resins indicate the viscosity of the polyvinyl acetate from which the resin was 25 made. The second digits indicate the extent to which acetate groups have been removed by hydrolysis. For example, Formvar 12/85 is made from a polyvinyl acetate having a viscosity of 12.0 cps (viscosity of a benzene solution containing 86 grams of polyvinyl acetate per 1000 ml. of solution, measured at 20~C.). Approximately 85 percent of the acetate groups have been replaced 30 with alcohol and formal groups.
Formvar resins can be described in general terms by their viscosity and solubility characteristics. Formvar 12/85 has the widest solubility range and is a medium viscosity type. A11 other types are more limited in solubility but are available in several viscosity ranges.
In Butvar resins, the acetate content is maintained at a low level and therefore exerts little influence on polymer properties. They are available .
. . , ,.
, : -in a variety of molecular weight ranges and types B-r~6 and B-79 have a lower hydroxyl content which permits broader solubility characteristics.
As a general rule, the substitution of butyral or formal groups for acetate groups results in a more hydrophobic polymer with a higher heat distoltion temperature. At the same time, the polymer's toughness and ad-hesion to various substrates is considerably increased. The outstanding ad-hesion of the vinyl acetal resins is believed to be a result of their terpolymerconstitution because each molecule presents the choice of three different functional groups to a surface and thus the probability of adhesion to a wide 10 variety of substrates is increased substantially.
Although polyvinyl acetal resins normally are thermoplastic and soluble in a range of solvents, they may be cross-linked through heating and with a trace of mineral acid. Cross-linking is thought to be caused by trans acetalization, but may also involve more complex mechanisms such as a re-15 action between acetate or hydro2cyl groups on adjacent chains. Generally, cross-linking of the polyvinyl acetals is carried out by reaction with various thermosetting resins such as phenolics, epoxies, ureas, di-isocyanates and melamines. Incorporation of a small amount of vinyl acetal resin into thermo-setting compositions will markedly improve toughness, flexibility and adhesion 20 of the cured coating.
Vinyl acetal films are characterized by high resistance to ~liphatic hydrocarbons, mineral, animal and vegetable oils (with the exception of castor and blown oils). They withstand strong alkalis but are subject to some attack by strong acids. However, when employed as components of cured coatings, 25 their stability to acids as well as solv~nts and other chemicals is improved greatly. The vinyl acetals will withstand heating up to 200F. for prolonged periods with little discoloration.
The carrier coating compositions of this invention may have a weight average molecular weight of between about 30,000 and about 270,000 30 and preferably between about 30,00û and about 45,000. Further, the coating compositions comprise from between about loO and about 21.0 percent polyvinyl alcohol, from between about 0 and about 2.5 percent polyvinyl acetate, and from between about 80.0 and about 88.0 percent polyvinyl acetal, all percentages being by weight of the composition. In addition7 these polymers 35 have a yield tensile strength of between about 5800 and about 7800 psi, and an apparent modulus of elasticity of between about 280,000 and about 340,000 psi, :, : ~ ,,: .- :
:
, .
' as determined by ASTM method D638-58T. As to the thermal properties, the polymers have an apparent glass temperature (Tg) of between about 48 C. and about 68 C. as determined by ASTM method D10~3-51.
In the preparation of the carrier materials of this invention, a 5 coating solution is applied to the carrier core particles to provide them with a thin, substantially continuous coating of polyvinyl acetal. The polyvinyl acetalcoating is applied to the carrier core particles by dissolving the coating material in a suitable solvent such as methyl ethyl ketone and dipping, tumbling or spraying the core particles with the coating solution. Preferably, 10 a fluidized bed coating process is employed as typically a more uniform coating is provided to the carrier core particles. In such a coating process, the core particles are suspended and circulated in an upwardly flowing stream of heated air so that the particles are sprayed by the coating material in a first zone. Then, in a second zone, the particles settle through an air stream of 15 lower air velocity where the solvent evaporates to form a thin solid coating on the particles. Successive layers of coating on the particles are obtained by recirculating them through the first and second zones of the fluid bed coat;ng apparatus.
Any suitable coating weight or thickness of polyvinyl acetal may be 20 employed to coat the carrier core particles. However, a coating having a thickness at least sufficient to form a substantially continuous film on the core particles is preferred because the carrier coating will then possess sufficient thickness to resist abrasion and minimize pinholes which may adversely affect the triboelectric properties OI the coated c~rrier particles, and also in order 25 that the desired triboelectric effect to the carrier is obtained and to maintain a sufficient negative charge on the carrier, the toner being charged positively in such an embodiment so as to allow development of negatively charged images to occur. Generally, for cascade and magnetic brush development, the carrier coating may comprise from about 0.05 microns to about 3.0 microns in 30 thickness on the carrier particle. Preferably, the coating should comprise from about 0.2 microns to about n.7 microns in thickness on the carrier particle becal~e maximum coating durability, toner impaction resistance, and copy quality are achieved. To achieve further variation in the properties of the final coated product, other additives such as plasticizers, reactive or non-35 reactive resins, dyes, pigments, conductive fillers such as carbon black,wetting agents and mixtures thereof may be mixed with the coating material.

~$Z~2 g In addition7 where the carrier core is a magnetizable material, it is possible to provide carrier particles having magnetic properties.
Following application of the coating to the carrier particles of this invention, it has been found that, when the carrier particles are mixed with a 5 conventional toner material such as one comprising a styrene/n-butyl metha-crylate copolymer and carbon black, the triboelectric charge generated on the carrier particles is of a positive polarity. Since such a triboelectric charge is unsuitable to provide satisfac-tory developed image print density with a negatively charged photoconductive surface, it has been found that when these 10 coated carrier particles are mixed with finely-divided toner particles contain-ing a triboelectric charge control additive, the carrier particles of this invention unexpectedly obtain negative triboeleetric charging values in the range of between about -15 to about -40 microcoulombs per gram of toner material. It was found that the triboelectric charging values of the thus 15 coated carrier particles are excellent to provide developed copies having high image print density, high resolution and low bacl~ground. In addition, the triboelectric charging values of the carrier particles remain stable over extended periods of milling.
Any suitable well-known coated or uncoated carrier material may 20 be employed QS the core or substrate for the carrier particles of this invention.
Typical carrier core materials are methyl methacrylate, glass, silicon dioxide, flintshot, ferromagnetic materials such as iron, steel, ferrite, magnetite, nickel, and mixtures thereof. An ultimate coated carrier particle having an average diameter in the range substantially 30 microns to substantially 191)00 25 microns is preferred because the carrier particle then possesses sufficient density and inertia to avoid adherence to the electrost~tic images during the development process. ~dherence of carrier particles to an electrostatographic drum is undesirable because of the formation of deep scratches on the drum surface during the image transfer and drum cleaning steps, particularly where 3D cleaning is accomplished by a web cleaner such as the web disclosed by W. P.
Graff, Jr., et al. in U.S. Patent 3,186,838.
Any suitable pigmented or dyed toner material may be employed with the carrier particles of this invention. Typical toner materials are gum copal, gum sandarae, resin, cumarone-indene resin, asphaltum, gilsonite, 35 phenolformaldehyde resins, resin-modified phenolformaldehyde resins9 meth-acrylic resins, polystyrene resins, epoxy resins, polyester resins, polyethylene resins, vinyl chloride resins, and copolymers or mixtures thereof. The par-ticular toner material to be employed depends upon the separation of the toner particles from the carrier particles in the triboelectric series. However, it ispreferred that the toner material comprise styrene and a lower alkyl acrylate 5 or methacrylate such as methyl methacrylate, n-butyl methacrylate, and 2-ethyl hexyl methacrylate in the form of mixtures or copolymers and terepolymers thereof. Among the patents describing toner compositions are U.S. Patent 2,659,670 issued to Copley; U.S. Patent 2,753,308 issued to Landrigan; U.S. Patent 3,û7(),342 issued to Insalaco; U.S. Reissue 25,136 to 10 Carlson, and U.S. Patent 2,788,288 issued to Rheinfrank et al. These toners generally have an average particle diameter in the range substantially 5 to 30 microns.
Any suitable pigment or dye may be employed as the colorant for the toner particles. Colorants for toners are l,vell known and are, for example,15 carbon black, nigrosine dye, aniline blue, Calco Oil Blue, chrome yellow, ultramarine blue, Quinoline Yellow, methylene blue chloride, Monastral Blue, Malachite Green Oxalate, lampblack, Rose Bengal, Monastral Red, Sudan Black BN, and mixtures thereof. The pigment or dye should be present in the toner in a sufficient quantity to render it highly colored so that it will form a clearly 20 visible image on a recording member.
Any suitable triboelectric charge controlling additive may ~e employed in the toner composition. Preferably, the additive will be one that enhances the positive triboeleetric charging characteristics of the toner par-ticles. Typical triboelectric charge controlling additives for this purpose 25 include cetyl pyridinium chlGride, cetyl pyridinium bromide, cetyl pyridiniumtolsylate9 cetyl alpha picolinium bromide, cetyl beta picolinium chloride, cetylgamma picolinum bromide, n-lauryl, n-methyl morpholinium bromide, n,n-dimethyl n-cetyl hydrazinium chloride, and n,n-dimethyl n~cetyl hydrazinium tolsylate available from Hexcel ~ompany; tetraethyl ammonium bromide 30 available from Eastman Kodak Company; spirit soluble black dyes such as Nigrosine SSB, 3-lauramidopropyl trimethylammonium methylsulfate, stear-amidopropyl dimetyl B-hydroxyethyl amrnonium dihydrogen phosphate, and stearamidopropyl dimethyl B-hydroxyethyl ammonium nitrate available from American Cyanamid Company; alkyl dimethyl benzyl ammonium chloride, 35 cetyl dimethyl benzyl ammonium chloride, and stearyl dimethyl benzyl ammonium chloride available from Hexcel Company; distearyl dimethyl ammonium chloride available from Ashland ~hemical ~ompany; di-isobutyl-cresoxylthoxyethyl dimethyl benzyl ammonium chloride available from Rohm and Haas Co.; and substituted imidazolines available from Ciba-Geigy Corporation.
Any suitable conventional toner concentration may be employed with the carrier particles of this invention. Typical toner concentrations are 1part toner with 10 to 200 parts by weight of carrier.
Any suitable well-known electrophotosensitive material may be employed as the photoreceptor with the carrier particles of this invention.
10 Well-known photoconductive materials are vitreous selenium, organic or in-organic photoconductors embedded in a non-photoconductive matrix, organic or inorganic photoconductors embedded in a photoconductive matri~, or the like. Representative patents in which photoconductive materials are disclosed include U.S. Patent 2,803,542 issued to Ul]rich, U.S. Patent 2,970,906 issued to15 Bixby, U.S. Patent 3,121,006 issued to Middleton, U.S. Patent 3,121,007 issued to Middleton, and U.S. Patent 3,151,982 issued to Corrsin.
In the following examples, the relative triboelectric values gen-erated by contact of carrier particles with toner particles are measured by means of a Faraday Cage. This device comprises a stainless steel cylinder 20 having a diameter of about 1 inch and a length of about 1 inch. A screen is positioned at each end of the cylinder; the screen openings are of such a size as to permit the toner particles to pass through the openings but prevent the carrier particles from making such passage. The Faraday Cage is weighed, charged with about 0.5 gram of the carrier particles and toner particles, 25 reweighed, and connected to the input of a coulomb meter. Dry compressed air is then blo~n through the cylinder to drive all the toner particles from thecarrier particles. As the electrostatically charged toner particles leave the Paraday Cage, the oppositely charged carrier particles cause an equal amount of electronic charge to flow from the Cage, through the coulomb meter, to 30 ground. The coulomb meter measures this charge which is then taken to be the charge on the toner which was removed. Next, the cylinder is reweighed to determine the weight of the toner removed. The resulting data are used to calculate the toner concentration and the average charge to mass ratio of the toner. ~ince the triboelectric measurements are relative, the measurements 35 should for comparative purposes be condueted under substantially identical conditions. Other suitable toners may be sllbstituted for the toner composition ' '.- :

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used in the examples.
The following examples, other than the control example, further illustrate and compare methods of preparing and utilizing the carrier particles of the present invention in electrostatographic applications. Parts and 5 percentages are by weight unless otherwise indicated.
EXAMPLE I
A developer mi2~ture was prepared by first applying a coating composition to steel carrier particles having an average diameter of about 100 microns. The coating composition comprised polyvinyl butyral commercially 10 available as Butvar 98 from Monsanto Plasties and Resins, St. Louis, Missouri.
The coating composition was diluted with methyl ethyl ketone and applied to the carrier particles in a fluidized bed coating apparatus. About 0.2 parts by weight solids of the coating composition was applied per about 100 parts of the carrier particles. After removal of the solvent, the coated carrier particles 15 were dried by heating in an oven at about 75C for about 30 minutes to remove any residual solvent. The coated carrier particles were cooled to room temperature and screened to remove agglormerated particles. About 100 parts of the screened carrier particles were mixed with about 3 parts of finely-divided toner particles to form a developer mixture. The composition of the 20 toner particles comprised styrene, methyl methacrylate, 2-ethylhexyl meth-acrylate, carbon black, and 3--lauramidopropyl trimethylammonium methyl-sulfate. The developer mixture was roll-mill and samples taken therefrom after about 1 and about 6 hours for measurement of the triboelectric eharge generated on the carrier particles as indicated above. The triboelectric value 25 was found to be about -26.3 and -~7.8 microcoulombs per gram of toner particles, respectively.
EXAMPLE II
A developer mixture was prepared by first applying a coating composition to steel carrier particles having an average diameter of about 100 30 microns. The coating composition comprised polyvinyl butyral commercially available as Butvar 98 from Monsanto Plastics and ~esins, St. Louis, Missouri.
The coating composition was diluted with methyl ethyl ketone and applied to the carrier particles in a fluidi~ed bed coating apparatus. About 0.15 parts by weight solids of the coating composition was applied per about 100 parts of the 35 carrier particles. After removal of the solvent, the coated carrier particles were dried by heating in an oven at about 75C for about 30 minutes to ' ''' ~

remove any residual solvent. The coated carrier particles were cooled to room temperature and screened to remove agglomerated partieles. About 100 parts of the screened carrier particles were mixed with about 3 parts of finely-divided toner particles to form a developer mixture. The composition of the 5 toner particles was the same as in Example I. The developer mixture was roll-mill mixed and samples taken therefrom after about 1 and about 6 hours for measurement of the triboelectric charge generated on the carrier particles as indicated above. The triboelectric value was found to be about -27.9 and-18.7 microcoulombs per gram of toner particles, respectively.
EXA~qPLE III
.
A developer mixture was prepared by first applying a coating composition to steel carrier particles having an average diameter of about 100 microns. The coating composition comprised polyvinyl butyral commercially available as Butvar 98 from Monsanto Plastics and Resins, St. Louis, Missouri.
15 The coating composition was diluted with methyl ethyl ketone and applied to the carrier particles in a fluidized bed coating apparatus. About 0.1 parts by weight solids of the coating composition was applied per about 100 parts of the carrier particles. After removal of the solvent, the coated carrier particles were dried by heating in an oven at about 75 C for about 30 minutes to 20 remove any residual solvent. The coated carrier particles were cooled to roomtemperature and screened to remove agglornerated particles. About 100 parts of the screened carrier particles were miz~ed with about 3 parts of finely-divided toner particles to form a developer mixture. The composition of the toner particles was the same as in Example I. The developer mixture was roll-25 mi~l mixed and samples taken therefrom after about 1 and about 6 hours formeasurement of the triboelectric charge generated on the carrier particles as indicated above. The triboelectric value was found to be about -26.6 and -19.0 microeoulombs per gram of toner particles, respectively.
EXAMPLE IV
A developer mixture was prepared by first applying a coating composition to steel carrier particles having an average diameter of about 100 microns. The coating composition comprised polyvinyl butyral commer-mercially available as Butvar 79 from Monsanto Plastics and Resins, St. Louis, Missouri. The coating composition was diluted with methyl ethyl ketone and applied to the carrier particles in a fluidized bed coating apparatus. About 0.15 parts by weight solids of the coating composition was applied per about 5~

100 parts of the carrier particles. After removal of the solvent, the coated carrier particles were dried by heating in an oven at about 75 C for about 30 minutes to remove any residual solvent. The coated carrier particles were cooled to room temperature and screened to remove agglomerated particles.
5 About 100 parts of the screened carrier particles were mixed with about 3 parts of finely-divided toner particles to form a developer mixture. The composition of the toner particles was the same as in Example I. The developer mixture was roll-mill mixed and samples taken therefrom after about 1 and about 6 hours for measurement of the triboelectric charge 10 generated on the carrier particles as indicated above. The triboelectric value was found to be about -25.0 and -30.8 microulombs per gram of toner particles, respectively.
EXAMPLE V
A developer mixture was prepared by first applying a coating 15 composition to steel carrier particles having an average diameter of about 100 microns. The coating composition comprised polyvinyl butyral commercially available as Butvar 79 from Monsanto Plastics and Resins, St. Louis, Missouri.
The coating composition was diluted with methyl ethyl ketone and carbon D black commercially available as Vulcan~X~-72 from Cabot Corporation, 20 Boston, Mass., was dispersed therein. The polymer solution-carbon black dispersion was applied to the carrier particles in a fluidized bed coating apparatus. About 0.2 parts by weight solids of ~he polymer and about 0.05 parts by weight solids of the carbon black was applied per about 100 parts of the carrier particles. After removal of the solvent, the coated carrier 25 particles were dried by heating in an oven at about 75C for about 30 minutesto remove any residual solvent. l`he eoated carrier particles were cooled to room temperature and screened to remove agglomerated particles. About 100 parts of the screened carrier-particles were mixed with about 3 parts of finely-divided toner particles to form a developer mixture. The composition of the 30 toner particles was the same as in Example I. The developer mixture was roll-mill mixed and samples taken therefrom after about 1 and about 6 hours for measurement of the triboelectric charge generated on the carrier particles as indicated above. The triboelectric value was found to be about -23.8 and-15.5 microcoulombs per gram of toner particles, respectively.
EXAMPLE VI
A developer mixture was prepared by first applying a coating

3~r~ q r k composition to steel carrier particles having an average diameter oi~ about 100 microns. The coating composition comprised polyvinyl butyral commercially available as Butvar 79 from ~onsanto Plastics and Resins, St. Louis, MissouriO
The coating composition was diluted with methyl ethyl ketone and carbon black commercially available as Vulcan XC-72 from Cabot Corporation, Boston, Mass., was dispersed therein. The polymer solution-carbon black dispersion was applied to the carrier particles in a fluidized bed coating apparatus. About O.ds parts by weight solids of the polymer and about 0.1 parts by weight solids of the carbon blaek was applied per about 100 parts of the 10 carrier particles. After removal of the solvent, the coated carrier particleswere dried by heating in an oven at about 75 C for about 30 minutes to remove any residual solvent. The coated carrier particles were cooled to r oom temperature and screened to remove agglomerated particles. About 100 parts of the screened carrier particles were mixed with about 3 parts of finely-15 divided toner particles to form a developer mixture. The composition of the toner particles was the same as in Example I. The developer mixture was roll-mill mixed and samples taken therefrom after about 1 and about 6 hours for measurement of the triboelectric charge generated on the carrier particles as indicated above. The triboelectric value was found to be about -24.7 and-21.5 20 microcoulombs per gram of toner particles, respectively.
EXAMPLE VII
A developer mixture was prepared by f irst applying a coating composition to steel carrier particles having an average diameter of about 100 microns. The coating composition comprised polyvinyl butyral commercially 25 available as Butvar 79 from Monsanto Plastics and Resins, St. Louis, Missouri.
The coating composition was diluted with methyl ethyl ketone and carbon black commercially available as Vulcan ~C-72 from Cabot Corporation, Boston7 Mass., was dispersed therein. The polymer solution-carbon black dispersion was applied to the carrier particles in a fluidi~ed bed coating 30 apparatus. About 0.7 parts by weight solids of the polymer and about 0.175 parts by weight solids of the carbon black was applied per about 100 parts of the carrier particles. After removal of the solvent, the coated carrier particles were dried by heating in an oven at about 75 C for about 30 minutes to remove any residual solvent. The coated carrier particles were cooled to 35 room temperature and screened to remove agglomerated particles. About 100 parts of the screened carrier particles were mixed with about 3 parts of finely-.: ' ''' ''~ " :

divided toner particles to form a developer mixture. The composition of the toner particles was the same as in Example I. The developer mixture was roll-mill mixed and samples taken therefrom after about 1 and about 6 hours for measurement of the triboelectric charge generated on the carrier particles as indicated above. The triboelectric value was found to be about -29.6 and-22.5 microcoulombs per gram of toner particles, respectively.
In summary, it has been shown that electrostatographic carrier particles coated with a polyvinyl acetal will provide carrier particles having negative triboelectric charging properties. These carrier particles possess such desirable negative triboelectric charging characteristics combined with e~cellent mechanical properties, low cost, and facile processability. ~urther9 the range of triboelectric charging values obtained is especially desirable and enables developer mixtures which provide maximum copy quality. Further, no post-treatment or fusing step is required in preparing the coated carrier particles of this invention such as with halogenated polymer coated carrier particles of the prior art.
Further, although specific materials and conditions are set forth in the foregoing examples, these are merely intended as illustrations of the present invention. Various other suitable thermoplastic toner resin compo-nents, additives, colorants, and development processes such as those listed above may be substituted for those in the examples with similar results. Other materials may also be added to the toner or carrier to sensitize, synergize or otherwise improve other desirable properties of the system.
Other mGdifications of the present invention will oecur to those skilled in the art upon a reading o~ the present disclosure. These are intended to be included within the scope of this invention.

Claims

WHAT IS CLAIMED IS:

1. An electrostatographic imaging process comprising providing an electrostatographic imaging member having a recording surface, followed by forming a negative-ly charged electrostatic latent image on said surface, contacting the image with a developer mixture consisting essentially of finely-divided toner particles electro-statically clinging to the surface of carrier particles, the toner particles consisting of a resin and colorant, and the carrier particles consisting of a core selected from the group consisting of iron, steel, ferrite, magnetite, nickel, and mixtures thereof, and having an average diameter of from between about 30 microns and about 1,000 microns, said core containing thereon an outer coating of polyvinylbutyral in a thickness of from about 0.2 microns to about 0.7 microns, thereby resulting in a negative triboelectric charge on the carrier particles, and a positive triboelectric charge on the toner particles, causing at least a portion of the finely-divided positively charged toner particles to be attracted to and deposited on the recording surface in conformance with the electro-static latent image to provide a developed image, trans-ferring the developed image to a suitable substrate, and permanently affixing the developed image to said substrate.